This invention relates to switched vertical deflection circuits for a television receiver.
In a switched vertical deflection circuit of the type disclosed in U.S. Patent Application Ser. No. 595,809, now U.S. Pat. No. 4,048,544, filed July 11, 1975, by Peter Eduard Haferl, entitled, SWITCHED VERTICAL DEFLECTION SYSTEM, horizontal rate energy, in the form of horizontal retrace pulses from a horizontal output transformer of a horizontal deflection circuit, charges a capacitor in parallel with a vertical deflection winding. A first switch, such as an SCR, couples successively smaller portions of the horizontal rate energy to the capacitor during a first part of the vertical trace interval and a second switch, such as another SCR, couples successively larger portions of the horizontal rate energy during a second part of the vertical trace interval. The voltage across the capacitor is integrated by the vertical deflection winding into a sawtooth vertical deflection current. The conduction of the two SCR switches is controlled by horizontal rate pulse width modulated pulses coupled from a modulator to the SCR gate electrodes.
At the start of vertical retrace, the second SCR switch which had previously been conducting is maintained in cutoff. The vertical deflection winding and the capacitor form a resonant retrace circuit. A disconnect diode coupled to the gate of the first SCR switch is reversed biased, maintaining the SCR in cutoff independent of the gating pulses generated by the modulator. With both SCR's nonconducting, resonant retrace of the current in the vertical deflection winding is accomplished. At the start of the subsequent vertical trace interval, the disconnect diode is no longer reverse biased. Pulse width modulated gating pulses to the first SCR enable the SCR to couple the horizontal retrace pulses to the capacitor for generating the sawtooth deflection current in the vertical deflection winding.
Both SCR's conduct relatively large amounts of current at the beginning and end of the vertical trace interval, respectively. Neither SCR conducts during the vertical retrace interval. Accordingly, loading of the horizontal deflection circuit by the switched vertical deflection circuit will be greatest at the beginning and end of the vertical trace interval, with substantially no loading occurring during the vertical retrace interval. Such load interruption during the vertical retrace interval may cause undesirable modulation of the horizontal deflection current and undesirable oscillations within the horizontal deflection circuit. These oscillations may appear, for example, in the "S" shaping capacitor or in the horizontal output transformer windings as the load impedance of the vertical deflection circuit abruptly changes. It is, therefore, desirable to provide a switched vertical deflection circuit in which undesirable oscillations within the horizontal deflection circuit are prevented.